1. FIELD OF THE INVENTION
This invention relates to magnetic memory cells and magnetic sensor elements wherein a nonmagnetic, resistive layer overlays at least part of a magnetic layer, and wherein the resistive layer is interposed between the magnetic layer and an electrical contact.
2. RELATED ART
Certain magnetic memories and magnetic sensors utilize thin films of ferromagnetic, magnetoresistive materials as key sensing elements.
Ferromagnetic materials are materials possessing permanent magnetic dipoles which exhibit a high degree of alignment at room temperature. The net magnetic moment M (or magnetization) of a ferromagnetic material is a measure of the alignment of the dipole moments in the material.
In forming ferromagnetic thin films, the orientation of M can be selected by exposing a ferromagnetic material to a unidirectional external magnetic field during deposition or annealing. The resulting uniaxial anisotropic magnetic film has what is referred to as an easy magnetic axis (aligned with the direction of the externally applied magnetic field), and a hard magnetic axis which is perpendicular to the easy axis.
Further, the magnetic thin film is magnetoresistive. That is, the electrical resistance of the film depends upon the orientation of the easy axis relative to the direction of current flow. The maximum resistance occurs when the magnetization vector and the current direction are parallel, and the minimum resistance occurs when they are perpendicular.
In magnetic memories, data storage lines are formed of the above described magnetoresistive thin films. Data is stored in binary fashion by utilizing a magnetic thin film deposited, as distinct cells, along a sense or bit line. The easy axis is often oriented along the bit line. If the magnetization of a cell is in a first direction along the bit line, the bit is defined as a 1; if the magnetization is in a second direction opposite the first direction, the bit is defined as a 0.
A conductive current strap, or word line, is typically disposed orthogonal to and overlaying the bit line. The word line is electrically isolated from the bit line.
The data condition of a cell is sensed or read by passing a sense current through the bit line and a word current through the word line. The magnetic field associated with the sense current interacts with and rotates the magnetization of the thin film within the plane of the thin film to an oblique position with respect to the easy axis. The rotated magnetization vector will be in a different position for a 0 than for a 1. The magnetic field associated with the word current will either increase or decrease the angle of rotation of the magnetization with respect to the easy axis, depending on the logic state of the bit being sensed.
A sense amplifier, connected across the bit line and responsive to the sense current, will detect a different electrical signal for a 0 than for a 1.
A write operation is similar to a read operation except that the magnitude of the sense and word currents are increased so that together the magnetic fields associated with the sense and word currents are sufficient to flip the rotated magnetization vector from one logic state to the other.
Some magnetic memories form the easy axis perpendicular (or transverse) to the longitudinal axis of the bit line. Read and write operation in that case are similar to those of the longitudinal cell.
Some magnetic sensors employ a sense element which is similar in structure to the magnetic memory bit line. That is, a magnetoresistive (ferromagnetic) thin film is formed into a strip having an easy axis. A sense current is passed along the strip. The presence of an external magnetic field due, for example, to current or the movement of a ferromagnetic object, will lead to interaction of the external magnetic field and the magnetization of the thin film strip. The change in resistance of the thin film due to the interaction is sensed by measuring the effect on the sense current.
In both magnetic memories and magnetic sensors, signal levels are typically very small, i.e. on the order of few millivolts or a few milliamps. Thus one must take care not to substantially reduce signal levels when modifying the device structure.
Both the magnetic memory and the magnetic sensor can be formed by thermal and vapor deposition techniques. Selected layers of insulators and metals are grown or deposited, and etched to form a solid state, monolithic device.
In the course of this processing, a magnetoresistive thin film is deposited. For magnetic memories, the thin film is typically deposited as a "sandwich" structure, i.e. first and second ferromagnetic thin film layers separated by a thin intermediate layer. The two ferromagnetic layers have their magnetization vectors antiparallel. The intermediate layer is selected and configured to break the exchange coupling between the two ferromagnetic layers. This sandwich structure results in good "flux closure", i.e. the magnetic field lines due to the magnetization of the sandwich are primarily confined to a closed path within the structure. Flux closure reduces demagnetizing effects due to the presence of free magnetic poles at the film edges and due to other nearby magnetic fields, and is important for the proper functioning of magnetic memories with densely packed cells.
The top ferromagnetic thin film in the sandwich structure is placed in electrical contact with a conductive lead at each end of the bit, so that the ends of adjacent bits can be joined by a good conductor to form a good current path for the sense current.
However, in forming the top ferromagnetic layer in the sandwich, one typically etches through an insulative layer to expose the top ferromagnetic layers and form a via in which the conductive lead will be deposited. Generally, ferromagnetic thin films readily oxidize. Thus exposure of the top thin film during device processing will result in a thin oxide layer on its upper surface, which will alter the electrical characteristics of the contact site in undesirable and often unpredictable ways. Further, since the top thin film is typically less than 1000 .ANG. thick (and often only a few hundred angtroms thick or thinner) it is difficult to control the etching with enough precision to avoid cutting deeply into the top layer or even etching through it. Deep cuts in the upper thin film can substantially adversely affect flux closure, sense current flow and device performance.
Likewise, in the case of a magnetic sensor utilizing either a single thin film ferromagnetic layer (or a sandwich structure similar to magnetic memory bit cells) vias are formed in an overlaying layer to expose the upper surface so that conductive contacts can be made to the film. Oxidation of the upper surface or etching deeply into the thin film cause similar problems in the sensor to those of magnetic memory cells.
Protective layers overlying the magnetic film could be employed, but if the protective layer is a good conductor, it could short out the electrical properties of the magnetic film and reduce the already low signal currents flowing in the magnetic films.
A thin film structure which could protect the magnetic thin film during processing, and afford good electrical contact to conductive leads while leaving sufficient signal levels within the thin film, is thus highly desirable.